For thousands of years, many attempts have been made to block the heat flow across surfaces of objects and structures for various reasons including better living environment. In fact, the development of good thermal insulation material has gained great strategic importance especially after energy crisis of the 1970s.
In recent years, China has been adding 1.7 billion m2 of new floor space on an annual basis (including urban and rural areas). In 2010, the total area of existing buildings in China was approximately 48.6 billion m2 (Building Energy Efficiency Policies in China—Executive Summary, July 2012, Global Building Performance Network. ISBN: 99-10-91655-01-9). Most of the existing buildings and 30% of the buildings under construction are high energy consumption structures. It is undoubted that a huge potential exists for thermal insulation materials in order for these buildings to achieve energy saving targets.
Objects constantly exposed to radiant energy, such as railway track, develop large thermal stresses which will shorten their working life. Reducing this temperature difference by coating with a thermal insulation material is a possible way to decrease the life cycle cost of these objects.
Central heat supply is provided to an area of 1.0108 billion m2 in China in 2005 (Chinese Statistical Yearbook of 20054) and, in Beijing alone, the length of pipelines in the heat supply network exceeded 17,000 kilometers in 2008 (Beijing Development and Reform Commission, 2009). This kind of structures enclosing materials with a large temperature gradient with its immediate environment would have a huge demand for thermal insulation materials.
Conventionally, low thermal conductivity is considered as the most important factor in choosing a thermal insulation material. As still air or other gases have low thermal conductivity, most of the thermal insulation materials tend to have a very high void content such that the bulk of the thermal insulation material consists of small bubbles of air.
Due to the high porosity of traditional thermal insulation materials, such as aerated concrete block, foam plastics, expanded perlite, foam cement, expanded polystyrene, most of them are not expected to have sufficient mechanical strength to support a large load. The material will lose its function when subjected to any accidental physical or chemical attacks. As a result, most traditional thermal insulation materials are not durable especially under abrasion and adverse environmental conditions such as moisture and acid gases in the atmosphere. The temperature and relative humidity will also affect the thermal conductivity of the traditional thermal insulation materials. Once the temperature or the humidity exceeds an acceptable range, the thermal conduction of the material will increase dramatically. Flammability is another significant problem with existing thermal insulation materials. Protection is always needed on the surface of thermal insulation materials to resist fire or prevent the release of toxic vapors under high temperature. This incurs additional complexity and cost on the thermal insulation installation.
The most significant problem with traditional thermal insulation materials is their ineffectiveness in resisting radiant heat. The radiant heat, in the form of electromagnetic waves, will cause molecules to vibrate and thereby causes increase in temperature. The wavelengths responsible for heat from sunlight are from 400 nm to 25 μm. Most common thermal insulation materials are capable of absorbing and transferring 90 percent of the radiant energy. In any way, the radiant heat will generate on the surfaces of the traditional thermal insulation materials or be transferred through the materials from one side to the other. The only thing that the traditional thermal insulation can do is to reduce the efficiency of energy transfer across the material. So the tradition thermal insulation is, generally, a kind of passive method in heat resistance.